Test effective permissions of file for user

Question

Is it possible to test effective permissions of a file for a specific user?

I normally do this by su user and then accessing the file, but I now want to test this on an user with no shell (i.e. a System user)

Answer 1

The sudo command can run anything as a particular user with the -u option. Instead of worrying about shells, just try to cat (or execute, whatever) your file as your target user:

$ sudo -u apache cat .ssh/authorized_keys 
cat: .ssh/authorized_keys: Permission denied

Answer 2

I've found you can use su -s <shellname> <username> to enter a specific shell as a specific user. You can then test file permissions as usual.

E.g.:

su -s /bin/bash Debian-exim
touch /etc/exim4/exim4.conf.template